In order to test the quality of the manufacturing processes used in the production of printed circuit board (“PCB”) and/or semiconductor assemblies, destructive and non-destructive mechanical strength tests are performed. Typically the testing is performed on a bond which is providing electrical or thermal continuity between two materials. For bonds that are of a suitable shape and size the test force can be applied by gripping, hooking or shearing one of the materials making the bond.
There are some bonds where this is not possible, typically where the bond is a ball or bump of solder on the surface of a PCB or a semiconductor substrate, but is of unusual shape or size or which is difficult to access using gripping jaws. As an alternative means of applying a test force, it is known to melt the bond, and then allow the bond to re-solidify around a test tool. The test tool can then be moved to apply a test force to the re-solidified bond. An example of this type of system used for pull testing is described in U.S. Pat. No. 5,641,913 to Watanabe.
In practice, this operating principle has been implemented by adapting existing bond testing machines designed for hooking or gripping bonds to perform a pull test. A machine currently used in this way is the Dage 4000 multifunction Bondtester, available from Dage Holdings Limited, 25 Faraday Road, Rabans Lane Industrial Area, Aylesbury, Buckinghamshire, United Kingdom. A test pin is attached to the hook ordinarily used for pull testing. The test pin is held in place by the hook, which is directly attached to a beam fitted with strain gauges to measure the force applied during the test. One end of the pin has a 90 degree bend formed which engages the hook and transfers the force to the tip of the pin. The system uses a cartridge heater inserted into a relatively large titanium block which mechanically supports a heater and a thermocouple. The hook is accurately aligned above the titanium heater block so that the straight portion of the test pin passes through a close fitting hole running through it.
The method of operation of this equipment is as follows. The sample to be tested is rigidly fitted into a work holder attached to a horizontally movable table. The operator uses joystick controls to move the specific test site on the sample directly under the test pin, typically using a microscope to achieve the required accuracy. The operator lowers the whole load cell and test pin assembly, which is mounted to a motorized vertical stage, using the joystick control until the tip of the test pin is resting on the top of the solder ball/bump to be tested. The test button is pushed which heats the probe, through the titanium heater block, to a pre-determined temperature. Once the solder ball/bump melts, the very tip of the test pin, under its own weight, drops into the molten pool of solder. When the desired temperature has been reached, the heater is turned off, which allows the test pin to start cooling and the solder to solidify. Once solidified, the pin, the solder and heater block are cooled more rapidly by means of a cold air jet directed at them. Once a predetermined temperature has been achieved the test pin is anchored into the body of the solder ball/bump and the destructive pull test can be started. The whole load cell assembly is automatically driven upwards which causes the pull hook to apply an axial load on the pin and therefore the bond. A beam in the load cell flexes and calibrated strain gauges measure the force. As the bond fails, the strain gauges see the force dropping off and the maximum force prior to failure is recorded. The recorded force is then stored in a database.
This bond testing apparatus and method suffers from a number of disadvantages in measurement accuracy, speed of operation, and usability, and it is an object of the present invention to overcome some or all of these disadvantages, or at least to provide a useful alternative.